1. Field of the Invention
The invention relates to bonded sol-gel alumina abrasive tools and particularly those bonded with a bond material that can be converted to a semi-crystalline ceramic bond.
2. Technology Review
A vitreous bonded abrasive product, such as a conventional grinding wheel, comprises three volume components: an abrasive particulate material which usually occupies about 35 to 50 volume %; a vitreous bond material that provides typically about 5 to 15 volume % of the total; and the balance of the volume is void space. The function of the bond material is to hold the abrasive particles in place so that they can do the abrading work. In a typical vitreous bonded product of the prior art the glass components are added to the abrasive particles and the mixture is heated till the glass components melt, fuse to form a glass, and then flow to the particle contact points to form a bond post that solidifies on cooling. This provides the rigid structure of the finished product. In a more recent method the glass bond material is formed separately as a molten mass, cooled to solidify and then ground up. This ground up material, know as a frit, is then mixed with the abrasive particles. The advantage of this procedure is that the heating step can be shortened, the bond composition is more uniform and the forming temperature can often be reduced.
It will be appreciated that the rigidity and strength of the products of the prior art are often determined by the bond posts. Glass, being an amorphous material, has a low strength, (about 40 to about 70 Mpa), by comparison with the abrasive particles. This low strength gives rise to premature release of grain and enhanced wear. Hence the grinding ability of vitreous bonded products is in theory limited by the strength of the posts. In practice, with most abrasives, such limitations were not very significant. Some more modern abrasives such as sol-gel alumina abrasives however are adapted to perform best under a heavy load and this puts the bond under considerable stress. Traditional glass bonds are often found inadequate under such conditions and there is therefore a need for vitreous based bonds with a greater ability to operate under high stresses.
It has been proposed that there might be advantage in the use of a glass-ceramic bond to bond abrasives. However it has not been found possible heretofore to ensure that the bond material is concentrated in the bond posts or in coating the abrasive grits. This of course is extremely inefficient and has not resulted in any commercialization of such glass-ceramic bonded materials in spite of the potential advantages that might be expected.
For example, Clark et al. proposed this in a paper entitled "A Novel Technique for Producing a Glass-Ceramic Bond in Alumina Abrasives", Am. Ceram. Soc. Bull., 65 [11] 1506-12 (1986). Clark et al. indicated that most glass-ceramic bonds tested lacked sufficient flow and spreading to bond well to alumina. For the one bond in Clark which achieved what was termed "a good degree of flow", the result was an abrasive product with a diametrical strength of only approximately 60% of the level for abrasive products made with conventional glass bonds.
The present invention provides significantly improved bond material which performs unexpectedly well when used in combination with sol-gel alumina abrasives. It has significantly greater strength than traditional bonds and is easily formed. Abrasive products comprising sol-gel alumina abrasives and such bond materials perform unexpectedly better than those made with prior art bonds or glass-ceramics and conventional abrasives. The bonds further can be used with a wide variety of abrasives and exhibit an impressive versatility in the kinds of abrasive products that can be made with them.